1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions.
2. Discussion of the Background
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction machines including at least two of these functions, include a charging device to charge the surface of an image carrier such as a photoconductor uniformly, a writing unit to direct writing light onto the surface of the image carrier to form an electrostatic latent image thereon, and a developing device to develop the latent image with toner into a toner image. Then, the toner image is transferred from the image carrier onto a sheet of recording media either directly or indirectly in a transfer process, after which a cleaning member such as a cleaning brush or cleaning blade removes any toner (hereinafter “residual toner”) remaining on the surface of the image carrier.
The charging device is conventionally a charging roller disposed to contact the image carrier, and a charging bias is applied to the charging roller so as to charge the image carrier to a given electrical potential. However, in this configuration, because the image carrier and the charging roller rotate while in constant contact with each other, the residual toner and lubricant applied onto the image carrier tends to adhere to the charging roller. The toner and lubricant adhering to the charging roller can inhibit a uniform electrical discharge between the image carrier and the charging roller, and thus the image carrier cannot be charged properly, which is hereinafter referred to as “charging failure”. Charging failure causes substandard images in which image density is uneven, toner is partly absent creating white lines, or line-like stains are present.
In view of the foregoing, several approaches described below, have been advanced to reduce the adherence of toner and lubricant to the charging roller.
For example, in certain known image forming apparatuses, the charging roller is disposed across a given small gap from the image carrier so as to reduce adherence of the toner and the lubricant to the charging roller. However, although the adherence of the toner and the lubricant to the charging roller may be smaller when the charging roller does not contact the image carrier, it is still possible that the toner may be electrostatically transferred from the surface of the image carrier across the small gap to the charging roller.
In another known image forming apparatuses, the cleaning blade to remove the toner and the lubricant from the image carrier is double-layered and includes an electroconductive portion. A DC (direct current) bias that is lower than a discharge start voltage at which electrical discharge is started between the image carrier and the cleaning blade is applied to the cleaning blade to electrostatically attract particles of external additive to toner mother particles. The polarity of the external additive is the opposite of the polarity of the toner mother particles, and thus the DC bias applied to the cleaning blade has a polarity identical to that of the toner mother particles. However, although the cleaning blade can attract the particles whose polarity is the opposite of the polarity of the toner mother particles, other particles can adhere to the charging roller.
Yet another known image forming apparatus includes the charging roller disposed to contact the image carrier, and a charge adjustment member that charges the residual toner to have a polarity identical to that of the bias to charge the image carrier before the charging roller charges the image carrier to a given electrical potential. This known image forming apparatus is a cleaner-less type without the cleaning member to clean the surface of the image carrier, and the development device collects the residual toner whose polarity has been adjusted. However, because the charging roller contacts the image carrier, the residual toner can physically adhere to the charging roller.
In view of the foregoing, there is a need to reduce the adherence of the toner, the lubricant, etc., to the charging roller so as to prevent charging failure as well as to attain satisfactory image quality for a longer time period, which the known image forming apparatuses fail to do.